1. Field of the Invention
The present invention relates to flexible circuits, and in particular to methods, systems, and devices for manufacturing flexible circuits in very high volumes and at very low costs.
2. Description of the Prior Art
Radio frequency identification device (RFID) technology is proliferating everywhere and into everything. Right now, a worldwide effort is stepping into high gear to replace the familiar universal product code (UPC) barcodes on products with RFID tags. The ink and labels used to print UPC barcodes is very inexpensive, and the costs of RFID chips and printed circuit antennas are under a lot of pressure to match them. Large, expensive items, of course, are not price sensitive to the cost of a typical RFID tag. But mass produced commodity items need tags that cost only a few cents.
The majority of printed circuit boards (PCB's) are made by depositing a layer of copper cladding over the entire substrate, then subtracting away the unwanted copper by chemical etching, leaving only the desired copper traces. Some PCB's are made by adding traces to a bare substrate by electroplating.
Three common subtractive methods are used to make PCB's. Etch-resistant inks can be screened on the cladding to protect the copper foils that are to remain after etching. Photoengraving uses a photomask to protect the copper foils, and chemical etching removes the unwanted copper from the substrate. Laser-printed transparencies are typically employed for phototools, and direct laser imaging techniques are being used to replace phototools for high-resolution requirements. PCB Milling uses a 2-3 axis mechanical milling system to mill away copper foil from the substrate. A PCB milling machine operates like a plotter, receiving commands from files generated in PCB design software and stored in HPGL or Gerber file format.
Additive processes, such as the semi-additive process starts with an unpatterned board and a thin layer of copper. A reverse mask is then applied. Additional copper is plated onto the board in the unmasked areas. Tin-lead and other surface platings are then applied. The mask is stripped away, and a brief etching step removes the now-exposed thin original copper laminate from the board, isolating the individual traces.
The additive process is commonly used for multi-layer boards because it favors making plating-through holes (vias) in the circuit board.
Circuit etching methods that use chemicals, coatings, and acids are slow, expensive, and not environmentally friendly. Mechanical etching has been growing rapidly in recent years. mechanical milling involves the use of a precise numerically controlled multi-axis machine tool and a special milling cutter to remove a narrow strip of copper from the boundary of each pad and trace.
Conventional laser etching of circuit traces is from the side with the metal to be etched. The metal, smoke, and debris goes flying directly in the path of the laser beam trying to do its work. The laser and its optics need frequent cleaning in order to maintain etching efficiency. But lasers can be a very fast, environmentally safe way to mass produce printed circuits, e.g., RFID's on flexible printed circuits (FPC) using DuPont's KAPTON polyimide film.